This is how I personally upload my world files.
1. Download a specific world from anywhere (PMC)
2. Drag and drop to Desktop.
3. Create and name a new WinRAR Zip Archive (WinRAR must be installed)
4. Open the downloaded world folder, and move region & level.dat into the named WinRAR Archive.
5. Open a browser and go to file.io
6. Upload your WinRAR Archive Folder to file.io
7. DO NOT HIT COPY LINK! (Reference Image)
8. Open your server, and do /mh >World Management > Upload a world > (name it) >Paste the link using CTRL + V
If all goes well, It would've upload with no problem. Hope this helps in some way.